1. Field of the Invention
The invention relates to a navigational aid for a vehicle subject to maneuverability and path constraints, such as an aircraft flying over territory having inhibited areas of overflight below a certain floor or reliefs and obstacles to be circumvented as they are close to or above its flight altitude.
2. Description of the Related Art
Various systems have been developed for warning the crew of an aircraft of a risk of collision with the ground. Some of these, such as terrain awareness and warning systems (TAWS), make a short-term trajectory prediction for the aircraft on the basis of flight information (position, bearing, orientation and amplitude of the velocity vector) that are provided by the onboard equipment, place it in a situation relative to a map of the overflown region extracted from a database of elevations of the terrain accessible on board and issue alarms to the crew of the aircraft each time that the short-term predicted trajectory would come into collision with the ground. These TAWS systems compliment their alarms with rudimentary recommendations of the kind “terrain ahead, pull up”. Some of these systems also give information about the level of collision risk that the reliefs and obstacles around the aircraft incur in the form of map which presents the reliefs or obstacles of the overflown terrain as strata of different colors depending on the magnitude of the risk incurred.
In this field, the Applicant has proposed, in French patent application filed on Sep. 26, 2003, under No. 03/11320, a navigational aid map representing the area of movement of a vehicle subdivided into regions shown in false colors and bounded according to the possibility of the vehicle to negotiate them and the time that the vehicle would take to reach them when they are negotiable, for example red in the case of nonnegotiable obstacles, yellow for remote accessible regions and green for close accessible regions. This navigation map is generated by means of a distance map constructed from a database of elevations of the terrain and of portions of the relief that are likened to obstacles to be circumvented. The distance map lists the curvilinear distances separating the instantaneous position of the vehicle relative to the various accessible points in the area of movement that is mapped, considering only the paths that avoid the obstacles to be circumvented. These distances are estimated by means of a propagation distance transform.
However, the known navigational aid maps indicating the nonnegotiable parts or parts that are prohibited from being negotiated as obstacles to be circumvented, give no information about the extent of the areas adjacent to the obstacles in which the vehicle has little freedom of movement in the short term owing to the proximity of an obstacle to be circumvented. However, this information is beneficial, most particularly in the case of an aircraft, as it may serve to indicate moments when flying becomes more tricky and when the crew must increase its attention.